1. Field of the Invention
This invention relates to an improved non-catalytic, porous-phase combustor comprising a multi-layered porous plate wherein the gas phase reaction and the actual combustion take place within the pores of a porous plate to provide higher combustion intensity and to provide a greater proportion of heat release by radiation and to an improved process for generating radiant energy. The improved combustor preferably comprises a porous plate having at least two discrete and contiguous layers, a first porous layer comprising a material having a low inherent thermal conductivity wherein fuel is preheated, and a second porous layer comprising a material having a high inherent thermal conductivity wherein combustion takes place. This combustor design generates radiant energy with improved energy efficiency, enhances the combustion intensity of the porous phase reaction of fuel and oxidant within the pores of the plate, reduces noxious pollutant emissions, and reduces flashback due to the inherent thermal conductivities of the porous plate materials.
2. Description of the Prior Art
In general, heat energy may be transmitted by conduction, convection or radiation. Heat transmission by radiation and utilization of infrared energy has many advantages over conventional heat transmission by convection and conduction, particularly for many types of industrial applications. The operation and construction of infrared burners and radiant heaters is relatively simple, and thus more economical than other type of heat generation means. The intensity of radiant heat may be precisely controlled for greater efficiency, and infrared energy may be focused, reflected, or polarized in accordance with the laws of optics. In addition, radiant heat is not ordinarily affected by air currents.
Conventional gas fired infrared burners utilize flame energy or hot gases to heat a radiating refractory or other material, and thereby produce an approximately flat flame on or above the radiating surface.
Several types of gas fired infrared generators are currently available. Radiant tube burners comprise internally fired radiation units wherein the radiating surface is interposed between the flame and the load. Surface combustion infrared burners have a radiating burner surface comprising a porous refractory. The combustion mixture is conveyed through the porous refractory and burns above the surface to heat the surface by conduction. A third type of gas fired infrared generator comprises a burner having a radiating refractory surface heated directly with a gas flame. A fourth type of infrared generator utilizes a porous catalyst bed to oxidize fuel at a low temperature in a low temperature catalytic burner. These types of gas fired infrared generators may be utilized in a variety of industrial applications, including space heating, drying operations, food processing, thawing materials and equipment, and miscellaneous processes, including condensation control, metal heating, chemical processing and glass industry applications.
U.S. Pat. No. 3,751,213 teaches a high intensity radiant gas burner having a ceramic honeycomb radiant element wherein combustion takes place within the cells of the honeycomb as well as in the combustion chamber. The material comprising the gas injection block, positioned just downstream from the combustion chamber, is chosen on the basis of its density, taking into account the uniformity of gas flow, thermal insulating properties, and durability of materials having various densities. Intrinsic thermal conductivities of materials of construction are not considered and, in fact, it is preferred that the entire structure comprise alumina, the intrinsic thermal conductivity of all elements therefore being the same.
Japanese Pat. No. 55025773 teaches an infrared burner having a honeycomb ceramic burner coated with an aqueous solution of magnesia-lithium silicate. The aqueous coating is then fired to form a conductive layer. Combustion takes place at individual pores on the surface of the conductive layer, and the conductive layer promotes even heat distribution.
U.S. Pat. No. 3,738,793 teaches an illumination burner having a layered porous structure, the layered pores maintaining a stable flame in a thoria-ceria illumination burner. Combustion does not occur within the pores of the combustor, but on the surface of the top layer.
U.S. Pat. No. 3,912,443 teaches a layered ceramic radiant gas burner wherein the outer radiating layer comprises a coarsely porous ceramic material and an inner gas distributing layer comprises a finely porous, highly permeable ceramic material.
U.S. Pat. No. 3,270,798 teaches a catalytic radiant burner having a lower density porous layer and a higher density porous layer, the lower density layer providing insulation and preventing flashback with flameless catalytic combustion in the catalytic layers.
U.S. Pat. No. 4,483,673 teaches a catalytic combustion burner having a heat insulation diffusion layer.
U.S. Pat. No. 3,833,338 teaches a surface combustion burner having a thermally conductive layer, such as foamed metal, to back the ceramic fiber layer to reduce the risk of flashback.
U.S. Pat. No. 3,947,233 teaches a free burner wherein the flames burn above the burner heat surface.
U.S. Pat. No. 4,090,476 teaches a radiation boiler containing a radiating substance providing flameless, non-catalytic combustion.
U.S. Pat. Nos. 4,047,876 and 4,154,568 teach catalytic combustion within a bed.